A rotation driving device is known, in which a motor, a reduction drive and an output shaft are provided coaxially. For example, a rotation driving device disclosed in Patent Document 1 (JP 09-177905 A) includes a rotation detector coaxial with a motor on an opposite side of the motor from a reduction drive. The above-described rotation driving device is connected to an output shaft and is capable of detecting rotation of a detection shaft inserted into a hollow-cylindrical motor shaft.
In the rotation driving device of Patent Document 1, a space for the rotation detector is provided in addition to spaces for the motor, the reduction drive and the output shaft. Therefore, a size of the rotation driving device in its axial direction may become large.